This study presents the structural response of a Submerged Floating Tunnel (SFT) subjected to earthquake loading and other relevant static loading types. The SFT is a competitive solution for crossings of deep seas, canals and fjords, as it is submerged in water and floats at a fixed submersion depth. It is supported by its buoyancy and tethers, connected to the seabed to maintain the structure at an even submersion level. Influence on the environment is low compared to other competitive structures, as the structure does not occupy any surface of the seabed, except for the tether foundations. Moreover, the environment has little influence on the SFT’s operability, as it is insensitive to harsh weather conditions. However, in areas of high seismicity, the SFT’s dynamic characteristics must be carefully tuned to the expected type of earthquake to optimize its performance. Previous research showed that special care should be taken of the transition between the SFT and land tunnels, as this joint has proven itself to be the main challenge.

The purpose of this research is to alter the transition structure between the SFT and adjacent land tunnels, such that an optimal dynamic response of a SFT is found to seismic events (Chapter 1). Initially, its performance is measured by comparing the seismic serviceability limit state (SLS) response to the static ultimate limit state (ULS) response. Subsequently, for more severe earthquakes, it is measured by comparing the SLS seismic stresses with maximum allowed levels of concrete prestress.

To analyse the SFT behaviour, a global model has been created of the SFT+land-tunnel system in Chapter 3, which is built by means of a linear-elastic finite element method in Python. It accounts for dynamic loads through the soil and tethers, as well as static loads by the structure's weight, traffic and buoyancy. The influence of stagnant water is accounted for by the Morison equation. Soil-Structure-Interaction (SSI) is incorporated using a Substructuring method. The properties of the soil and tethers are found in local sub-models, after which the total structural response of the global model is solved in the frequency domain. Later, its response is transformed to the time domain to obtain a time-series of displacements and forces. Its dynamic characteristics are studied by comparing Fourier spectra with SFT natural frequencies, which gives insight in the influence of design choices on the time-domain response.

The global model is validated with a replica model in the finite element software DIANA FEA (Chapter 4). A realistic case study is defined based on a previous TEC project in Chapter 5 and later applied in a parameter study in Chapter 6. Here, the effects of various design choices with respect to the transition structure are monitored using 3 earthquake signals. 5 standard end-joints and 2 special end-joints with seismic base isolation and viscous dampers are tested. The effects of Multi-Support-Excitation is studied by letting earthquakes horizontally approach the SFT with various wave speeds and angles of attack. Finally, the effect of SSI is compared with a non-SSI case, to see its contribution to the research.

In Chapter 7, the results are discussed and in Chapter 8 a conclusion is provided together with recommendations for both TEC and future research.

Offshore wind energy is one of the emerging renewable technologies toward a zero-carbon emission society. Historically, offshore wind farms have been developed in the North Sea, but nowadays wind farms are expanded to other regions such as North America and Asia, which are seismic prone.
In the design of Rotor-Nacelle-Assembly (RNA) and support structure for offshore wind farms, the number of design positions is reduced by using representative design positions (i.e. clustering). The number of designs is directly related to the efficiency of the design work, so it is worthwhile to minimize it by wisely selecting the representative position. However, such a clustering methodology has not been systematically developed in seismic design due to the limited number of projects and researches in seismic regions.
As for seismic analysis, there are several challenges to develop an efficient clustering strategy. First, higher (and multiple) vibration modes are excited in the system consisting of the wind turbine itself in interaction with the soil. Second, blade vibration (and blade-tower coupling effect) has a significant role in the output forces. Third, energy intake from the seismic ground motion is dependent on the position due to soil amplification. Considering the problem statement above, this research develops a clustering strategy for seismic analysis to optimize the design process on RNA and support structure for offshore wind farms. To achieve this goal, as a first step, a simplified calculation model is created to efficiently carry out the sensitivity studies. A combination of point masses with Euler-Bernoulli beam is used for RNA model while lumped mass at the tower top is conventionally used in past researches, which allows to properly represent the blade-tower coupling effect contrary to most of past researches. Considering modal participation mass, higher vibration modes are truncated, and principle vibration modes are detected, which improves the computational efficiency. Both the frequency domain method and the response spectrum method are developed and compared with proven software, BhawC, to verify their accuracy. Then, a sensitivity study is carried out by using the developed model. Through the sensitivity study, the system property is approximately quantified as a form of ”scaling factor”. Combining the scaling factor with acceleration response spectrum, the maximum bending moment for an arbitrary position (i.e. arbitrary water depth, soil condition and bedrock depth) can be quickly estimated. Subsequently, the positions in a wind farm are sorted into groups with similar load trends based on the estimated bending moment; this is the basis of the clustering strategy proposed in this thesis. Finally, the strategy is applied to virtual wind farms and its applicability is verified. Although it shows some errors in the estimation of the maximum bending moment itself, the accuracy is considered satisfactory to detect the most loaded position (i.e. design representative position) and categorize the positions which have similar load trend, which are one of the main requirements for a reliable clustering methodology. Also, the strategy is applied to more realistic soil condition, such as inhomogeneous soil and its applicability and limitation are investigated.

A railway turnout, which is a combination of a switch and a crossing is an important component in the railway network to divert trains. Due to the change in contact between the wheels and the rails as the train negotiates a turnout, severe impact loading occurs in two-directions: vertically and laterally. These cause negative environmental impacts in terms of ground vibrations. Therefore, understanding the ground response due to a railway turnout qualitatively and quantitatively is essential for soil surface in the vicinity. This thesis focuses on qualitatively and quantitatively describing the ground response through a dual load model setup through semi-analytical approach. A two-dimensional load on an Euler Bernoulli beam which is placed on a three-dimensional half-space is considered to describe the ground response. Followed by which, a load spectrum defining the interaction between the wheels and the rails is considered and the ground response is analysed. Measurements from the vicinity of a turnout is also analysed in order to compare the performance of the model to that of the real-life scenario. It is further concluded that qualitatively the ground response caused due to impact like loading that occurs at a crossing, and the presence of lateral loading is able to be explained by the model. Quantitatively, to analyse the response recommendations are to be followed to acquire the accurate load spectrum along with considering the complex track geometry for the lateral loading.

Liquid storage tanks are used worldwide to contain liquids such as drinking water, fuel and chemicals. Loss of containment during a seismic event can add to an earthquake’s destructiveness by posing a fire hazard, spilling toxic substances or reducing the supply of drinking water. It is therefore important that they remain operational at a pre-defined level of functionality during and after an earthquake event. Two effects that are recognized to play a role in the seismic behavior of liquid storage tanks are fluid-structure interaction and soil-structure interaction. Numerical models have the ability to describe the dynamic behavior of each domain accurately, but they tend to be time consuming compared to many of the available simplified analytical models. While the latter can be useful for design purposes, they do not necessarily yield accurate solutions to the complex system at hand. In this thesis, a semi-analytical model is put forward based on previous work by Canny (2018) and Molenkamp (2018) that combines both fluid-structure and soil-structure interaction with a dynamic substructuring approach. The model offers a computational efficiency and ease of use comparable to the aforementioned analytical models, but the only sacrifices to accuracy are the limitations imposed by the underlying assumptions and a number of discretizations. The tank, soil and fluid domains are treated separately. The tank is considered with Love's thin shell theory and the soil as a visco-elastic continuum. The fluid is assumed to be incompressible, irrotational and inviscid, allowing the application of linear potential theory. The solutions to the homogeneous equations of Love's shell theory, describing the behavior of the tank's bottom plate and wall segments, are found as eigenfunction expansions. By satisfying the boundary and interface conditions of the different tank parts, an eigenvalue problem is formulated that leads to the eigenfrequencies and mode shapes of the tank. The fluid velocity potential is rewritten as the superposition of three fluid velocity potentials that each satisfy a set of conditions regarding the velocity continuity at the wall and at the plate, and the free surface condition at the fluid surface. The velocity potential in addition needs to satisfy Laplace's equation and solutions are found again as an expansion of eigenfunctions that represent the fluid's modes. For the soil, Green's influence functions for uniform horizontal and vertical loads on a circular area at the soil surface have been used to assemble a dynamic soil stiffness matrix. The influence functions are again in the form of an eigenfunction expansion that is the solution to the homogeneous wave equation describing the soil's motions. Through satisfaction of a set of conditions at the fluid-structure and soil-structure interfaces, respectively velocity and stress continuity, and displacement and stress continuity, a final set of equations is obtained. The only unknowns remaining in this set are the modal amplitudes that after solving can be used to linearly combine the mode shapes to give the full dynamic behavior of the tank-fluid system. Motions and stresses at the soil surface can be found with the dynamic soil stiffness matrix. The dynamic soil-structure interaction is taken into account in the model by considering all wave fields present in the soil, namely the free-field (or incident), the scattered and the radiated wave field. Subsequent satisfaction of the soil-structure interface conditions ensures that soil-structure interaction is properly considered, and it yields a simple result in which the usually known free-field wave field can directly be applied in the excitation term of the plate's equations of motion.
Results are obtained in the frequency domain and can be transformed to the time domain with the inverse Fourier transform. Output can be obtained similar to the output of FE models with stresses and displacements at each location of the tank domain. In the fluid domain, velocities can be found as well as related variables, such as the fluid pressures at the fluid-tank interfaces and the fluid elevation at the fluid surface (sloshing). Regarding the soil, stresses can be obtained at the soil-structure interface and displacements at the soil surface. Besides, the model can give more insight in the effects of SSI, compared to a tank-fluid system founded on a rigid soil. Limitations of the model include the inability to model nonlinear effects, as a result of the linearity of the model, so that nonlinear failure mechanisms as buckling cannot be modeled, or nonlinear stress-strain relations. At the same time, nonlinear effects are not extensively covered in the other available analytical methods either. With regard to engineering practice, roofs, ring stiffeners and anchors are commonplace, but have yet to be included in the model. Similarly, the capacity to model multiple soil layers would bring the model a step closer to reality. Besides more accurate modeling of the soil composition at a greater depth, it would admit a more realistic model of the soil directly underneath the tank, which is often improved in case of initially unsuitable grounds. Before the model can be applied in practice however, it needs to be validated, for example with a FE model or experimentally. To increase the model's competitive advantage of computational efficiency, measures can be taken to improve it. The greatest improvements can be made in the assembly of the dynamic soil stiffness matrix by reducing the number of elements of the soil-structure interface. With the applied axisymmetric discretization, many of the matrix entries are the same, so that only a small number actually needs to be computed. However, a large number of very small elements is concentrated in the center of the soil-structure interface that does not add to the accuracy of the solution. Other discretizations could reduce computation time by reducing the number of elements and thereby the size of the dynamic soil stiffness matrix.